This invention relates to the field of fat rendering and more particularly to methods and compositions for improving the separation of inedible fat from process water and solid waste.
Fat rendering facilities have conventionally employed several methods for separating water from inedible fats and solids in the processing of waste resulting from the rendering operation, after the removal of the melted edible fat. In currently employed processes the waste, after separation of edible fat, includes inedible fat, water, and solids. Most rendering facilities use some type of holding tank or clarifier. Some of these clarifiers, or holding cells, use skimmer technology to remove fat and solids from several waste water streams throughout a typical plant. All these waste streams are brought together into such a holding cell. These waste streams co-mingle in the holding cells and tend, after time to roughly separate into three distinct layers or phases. The top-most layer contains largely fat or grease, together with water and some free fatty acids and salts of free fatty acids. The middle, and by far the largest phase, is the water layer generated from the water used in the process. This layer will also contain most of the same materials found in the top layer in different proportions. Finally, solids or sludge represents the bottom layer.
There are two primary purposes of these holding cells. First, removal of the fat and inedible tallow which, after a sufficient period of time rises to the surface where it can be removed by skimming which can then be sold. Secondly, it is necessary to perform the separation efficiently in order to discharge the waste water at an acceptable level of purity. Normally, if the retention time of the holding cell is long enough, then the separation of these three phases is improved to an acceptable level. This can and has been accomplished by increasing the capacity of the holding cells. This has, in the past, been the most used method of increasing the retention time.
In most holding cells or clarifiers, the subsequently separated fat layer, water layer and sludge layer are characterized, individually, with respect to the respective disposition of each layer. For example, the quality of water (middle phase) is generally measured by its Biological Oxygen Demand (B.O.D.) and the Suspended Solids (S.S.) in the water. The measured B.O.D. values and S.S. values (usually expressed in weight per unit volume, i.e. mg/l) are used to determine the acceptability and cost of discharge. For most water discharges (river and sewer), the cost to discharge the effluent is, among other things, also dependent upon the quantity of water; however, this is essentially constant from day to day when the processing or rendering plant is operating. Other measurements and restrictions are used occasionally in determining the quality of water depending on the local municipalities such as temperature, chemical oxygen demand (C.O.D.), or ammonia content. Generally, if one lowers either the B.O.D. or S.S. of the water, then the charge for discharging the water would be reduced. It is not economically feasible at the present time for some industries, including fat rendering plants, to eliminate the B.O.D. and S.S. values in water completely.
The sludge in the bottom layer is usually pumped or augered out of the plant and taken to one of many large lagoons. These discharge lagoons are alternated in such a fashion that the solids can be air dried and disposed of in a land fill operation.
The skimmings obtained from the top layer are cooked and sold as inedible tallow. In continuous rendering systems, the tallow can be blended with other fat prior to cooking in a cooker. In a batch rendering operation it may be cooked and sold separately or blended with other fat.
The tallow obtained, i.e. flotation grease, from the holding cells is generally of poor quality and represents from 2 to 10 percent of the total inedible tallow for a typical beef packing operation. Although, this percentage is small; it influences the quality of the entire inedible tallow considerably if it is blended.
Color and the free fatty acid (FFA) content of the tallow from the skimmings are the two primary criteria used in determining the quality, and therefore, the value of the tallow. There are several other measurements that can be used but are less significant. Furthermore, color (F.A.C.) and F.F.A. content correlates to one another. A dark color generally means a high F.F.A. content. It is because of this relationship that the determination of the F.F.A. is used to evaluate the value of the skimming or total inedible tallow. Under normal operating conditions the F.F.A. of flotation grease can vary between 3 and 85 percent for most facilities, which when higher than about 4 percent is undesirable.
In addition the quality of the water discharged from the holding tanks generally contains sufficient soluble impurities and suspended solids to render direct discharge costly or impossible without further treatment.